This is a continuation of studies designed to evaluate the efficacy and safety of 131 Iodine-labeled anti-CD20 (131 I-anti-B1) as therapy for low-grade and transformed B-cell lymphoma. Patients will receive an imaging and therapeutic dose of radiolabeled antibody and will be followed for response, duration and toxicity. Patients will receive the imaging dose as outpatients, and the therapeutic dose as inpatients for 3 to 5 days.